Functionality triggered by a mouse click / scroll with ALT modifier key is not working on L(Ubuntu), LXQt

[pehar1]

Is there an existing issue for this?

• I checked and did not find my issue in the already reported ones

Describe the bug

Alt+click to clear and start over does not work when refining a selection (red overlay) on L(ubuntu) 24.04, LXQt, self compiled 5.5.0+659~g6fb9ee1aeb. Also a selection (red overlay) is not cleared when the module is reset.

To double check I replaced in develop/masks/object.c
line 1156 GDK_MOD1_MASK by GDK_SHIFT_MASK and
line 1189 GDK_SHIFT_MASK by GDK_MOD1_MASK
and now I can clear the selection but no longer exclude areas.

And this (line 1156)
if(gui->creation && which == 1 && dt_modifier_is(state, GDK_SHIFT_MASK + GDK_CONTROL_MASK))
allows me to clear with CTRL+SHIFT+click

So the code is OK but ALT modifier ( GDK_MOD1_MASK ) does not work !
In https://docs.gtk.org/gdk3/flags.ModifierType.html one can find :

GDK_MOD1_MASK
The fourth modifier key (it depends on the modifier mapping of the X server which key is interpreted as this modifier, but normally it is the Alt key).

It seems reasonable to me to use other modifier(s) where the key mapping is unambiguous.

Steps to reproduce

1. Open image in darkroom
2. Open module Exposure
3. Select Drawn mask
4. Click Icon for Object mask, wait until image is analyzed
5. Select area with brush (Click) -> area is shown with red overlay
6. ALT+Click to clear this selection -> nothing happens.

Expected behavior

Clear the selection with other modifier(s), clear the selection when module is reset

Logfile | Screenshot | Screencast

No response

Commit

No response

Where did you obtain darktable from?

darktable.org / GitHub release

darktable version

5.5.0+659~g6fb9ee1aeb

What OS are you using?

Linux

What is the version of your OS?

L(ubuntu) 24.04

Describe your system

OS : Linux 6.8.0-106-generic / Ubuntu 24.04.4 LTS
Memory : MemTotal:       65759236 kB
Graphics Card :     Product Name                                       : NVIDIA GeForce RTX 2060
Graphics Card : Driver Version                                         : 580.126.09
OpenCL installed :   Device OpenCL C Version                         OpenCL C 1.2 
OpenCL activated : yes
Xorg : Version: 1:7.7+23ubuntu3
Desktop : LXQt
GTK+ : 3.24.41
Glib : ldd (Ubuntu GLIBC 2.39-0ubuntu8.7) 2.39
gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0

Are you using OpenCL GPU in darktable?

Yes

If yes, what is the GPU card and driver?

No response

Please provide additional context if applicable. You can attach files too, but might need to rename to .txt or .zip

No response

[andriiryzhkov]

GDK_MOD1_MASK is X11's Mod1 modifier, which should map to Alt but depends on the X server's modifier table. On your system (LXQt), Alt appears to be assigned to a different Mod slot.

You can verify with:

xmodmap -pm

If Alt_L/Alt_R are not listed under mod1, fix it with:

xmodmap -e "add mod1 = Alt_L Alt_R"

To make it permanent, add that line to ~/.Xmodmap.

[pehar1]

From your pixls comment : GDK_MOD1_MASK is used for Alt in many other places in darktable - for example, Alt+drag on an ellipse mask toggles border resizing, and Alt+scroll in several modules (exposure ...

Alt+drag on an ellipse mask toggles border resizing -> does not work
Alt+scroll in several modules (exposure ... -> does not work, changes the active LXQt-Desktop instead

xmodmap -pm
xmodmap:  up to 4 keys per modifier, (keycodes in parentheses):                                                
                                                                                                               
shift       Shift_L (0x32),  Shift_R (0x3e)                                                                    
lock        Caps_Lock (0x42)                                                                                   
control     Control_L (0x25),  Control_R (0x69)                                                                
mod1        Alt_L (0x40),  Alt_L (0xcc),  Meta_L (0xcd)                                                        
mod2        Num_Lock (0x4d)                                                                                    
mod3        ISO_Level5_Shift (0xcb)                                                                            
mod4        Super_L (0x85),  Super_R (0x86),  Super_L (0xce),  Hyper_L (0xcf)                                  
mod5        ISO_Level3_Shift (0x5c)

Trying to add Alt_R ( xmodmap -e "add mod1 = Alt_R" ) does not change anything, mod1 line stays the same, behavior stays the same.
Side note : the keyboard does not contain an ALT key on the right side, only ALT on the left.

My impression is that the problem isn't with the key mapping, but rather that key presses and mouse clicks with the ALT modifier are “intercepted” by LXQt, so they never actually reach darktable.

I haven't changed any of the default LXQt settings provided by L(ubuntu) in this regard. So this is likely to affect everyone who uses the LXQt desktop environment.

Two points remain :

• It seems reasonable to me to use other modifier(s) where the key mapping and usage is unambiguous, unlike the ALT modifier.
• Clear the selection when module is reset. That would be the expected behavior and would also serve as a partial workaround for the ALT issue

[andriiryzhkov]

This is system wide problem, but for AI object mask we can make life simpler with fix #20603.

[pehar1]

system wide problem

Yes, I’ve already tried a few combinations in a Java application I wrote myself. In many cases, the ALT modifier works—for example, ALT+1 (zoom to 100%) or ALT+3 (fit to screen) in darktable. But ALT+mouse seems to be a general issue in LXQt. In any case, thank you very much for the fix.