KDE's window-specific settings feature gives you a fine level of control over the way windows behave. It lets you change settings such as minimum/maximum size, position, overlay, and transparency. The GIMP uses multiple windows to select tools, tool options, and other items used in the process of image editing. By changing the default behavior of the various windows used by GIMP with KDE's window-specific settings, you can improve the GIMP interface.
Before getting into window-specific settings, you can use the GIMP's own window features to preserve your favorite layout. Open the GIMP, preferably in its own desktop window. Position the toolbar where it is out of the way and resize it to your preference. Open up any other dialogs you use and position and resize them too. Here, I have placed the toolbar on the left side and resized it to take up less space, and opened the Tool Options, Layers, and Brushes dialogs and placed them on the right side of the screen.
After everything is set up to your liking, go to the GIMP main window and click File -> Preferences -> Window Management. Uncheck "Save window positions on exit" and click Save Window Positions Now. From now on, each time the GIMP opens, the current position of the open windows will be restored.
To automatically resize the edit window when zooming in or out, or when scaling an image, Click File -> Preferences -> Image Windows, and check "Resize window on zoom" and "Resize window on image change."
Large images or images that are zoomed in can sometimes cover the GIMP toolbar and tool options windows. You can change this behavior so that specific GIMP windows will not be overlaid by the active window, making the toolbar and other dialogs always available on the screen, by using a setting called Keep Above.
Next, resize the Control Panel window so that you can see the GIMP toolbar, and use the KDE Control Panel to modify window-specific settings. How you open that screen depends on how your version of KDE is set up. Some distros use KMenu -> Control Panel -> Desktop -> Window-Specific Settings; others might use KMenu -> System Settings -> Window Behavior -> Window-Specific Settings. Click on the New button. The window that opens contains a Detect button. When you click it, a crosshair cursor will appear. When you then click in a window body, KDE obtains information about the window that it uses to automatically fill in information it needs to uniquely identify the window. Click the crosshair cursor anywhere in the GIMP toolbar window body, but not on the titlebar.
KDE then displays information about the selected window, such as class, role, type, and title. It identifies the window you detected uniquely so that you can change its behavior. Because the GIMP uses multiple windows, you have to further identify the toolbar in order for KDE to control it. Click on the setting to "Use window class and window role (specific window)," then click OK. This allows KDE to control just the toolbar window in the GIMP and not all of the other GIMP windows.
Now you can edit your window-specific settings. The first two tabs of the settings screen, Window and Window Extra, will have most of their information filled in by the Detect feature; just fill in a description on the Window tab. You can make changes in the other tabs, labeled Geometry, Preferences, and Workarounds, which let you control such items as positioning, window size, and whether a window should be displayed on the taskbar.
You'll find the Keep Above setting in the Preferences tab. Click next to it, and select Force from the dropdown box to its right, and select the checkbox next to Force. Click OK to save your changes, then Apply to make them active.
Your toolbar should not be overlaid now, even when editing large images that fill the screen. If this setting doesn't take effect immediately, restart the GIMP.
Other GIMP windows to modify
I find it handy to keep the Tools Options window available at all times. You can repeat the process above for that window to keep it visible. When you modify Tools Options, the Layers and Brushes dialogs are also affected. That's because the Tools Options window is in the Dock Windows class, and the Layers and Brush windows are part of that class too.
Also on the Preferences tab is a checkbox called Skip Taskbar that will keep all of the GIMP windows from appearing on the taskbar. Only the main GIMP window, the Toolbar, will appear on the taskbar. This saves room on the taskbar, and Alt-Tab won't show a long list of items. The various windows will already be on the screen because you set Keep Above.
After making those changes, GIMP behaves the way I want. My toolbar and tools options are never overlaid. My taskbar is clean, with only two instances of taskbar buttons, the main GIMP toolbar and the edit image window. I've modified all of the other GIMP dialogs the Skip Taskbar setting.
The order the Windows-Specific Settings appear is not important. The order is used only to group similar settings for convenience. You can move a setting up or down using buttons on the right of the window.
There are many window-specific settings that you can change in KDE to tailor the GIMP, or any application. By modifying various settings, you can get GIMP to behave the way you want and better suit your work style. Experiment with the various settings until you get the perfect combination.